Basement smells bad. Look for cat poops, change litter.
When you go into a person's house, and you smell that wash of cat smell, it's the human's fault, not the cat's. Cats want everything to be clean around them. They want where they live to be clean, they clean themselves, they want a clean litter box.
I get asked a lot why cats mess outside the litter box. The simplest thing is if you have more than one cat and you have two litter boxes, don't put them right next to each other.
Every time I pass a litter box I clean it.
I've had cats all my life and obviously loved them, but the litter box, and the having to always get a house sitter, they're just too - they're too rigid. Cats are too needy somehow.
Different cats don't like certain litter. They also don't like an unstable floor, no animal like's unstable floor. So if you put a thin piece of plastic down under a litter box and the cat walks on it and starts to slip, they don't like that. Any animal doesn't like an unstable floor.
